It is known that if pure methyl methacrylate is polymerized, the volume it occupies will shrink typically by about 20% to about 22%. There are also exothermic effects and acceleration in the rate of polymerization which make the reaction difficult to control. Accordingly, it has become common commercially to prepolymerize a portion of the monomer and prepare a solution of such prepolymerized methylmethacrylate in additional monomer so the casting, or final polymerizing, process can be more readily controlled and its ultimate size and shape anticipated. An example of a manner of use of such a syrup is illustrated in Hellsund's U.S. Pat. No. 3,371,383 and Opel et al U.S. Pat. No. 3,376,371, which describe the continuous casting of wide sheets of acrylic between two stainless steel belts appropriately moved and temperature-controlled while they are spaced apart with the polymerizing material contained between them. Such casting, and stationary or cell casting, has been performed with minor amounts of other monomers in the methyl methacrylate, such as ethyl or butyl acrylate, diunsaturated cross-linking agents, and the like.
Various organic and inorganic additives or fillers are and have been used in resinous articles for decorative, strength, fire-retardant, economic and other reasons. Pigments may be added for color, glass fibers for strength, aluminum trihydrate for fire retarding, and any inexpensive space-occupier for economic reasons. Ground polyester resin was added by Ross and Stevens to a polyester matrix (see Reexamination Certificate 634th B1 4,433,070). However, the particulate resin apparently remained at the same hardness throughout, without absorbing any of the matrix resin. Ground acrylic resin is used in a "molding mix" in Cunningham's U.S. Pat. No. 3,050,785 in a polymethylmethacrylate matrix of a particular type of tacticity; no instructions are given as to the preferred method of use and nothing is said as to the ability of the particular kind of PMMA to absorb the PMMA of the matrix. Acrylates of certain glycols are employed in Cohen et al U.S. Pat. No. 4,414,278; they are used because they are not swellable, however, contrary to the present applicants' objectives. Stevenson's U.S. Pat. No. 4,048,257 describes the use of lightly cross-linked polyester beads as components of thermosetting molding materials; certain methylmethacrylate mixtures are also described and a method of measuring swellability is given.
It may also be considered relevant to the background of this invention that impact resistance has been imparted to polymethylmethacrylate sheet and other products by incorporating into the acrylic sheet an effective amount of an impact modifier additive which is in the form of small beads or particles made of acrylic esters but also including butyl acrylate or other higher acrylate to form a rubbery component in the particle. See, for example, Hwa et al U.S. Pat. No. 3,661,994 and Ferry et al U.S. Pat. No. 3,985,703. Such special rubbery acrylic particles are typically made in aqueous suspension specially for imparting impact resistance and are not known for their ability to swell by incorporation of the matrix solvent monomer as in the present invention.